0.1 Meteorological instruments: rain gauges, temperature, relative  (Page 2/4)

2.2.2. Maximum thermometer uses mercury, as more viscous; has constriction so that as mercury goes up it can’t come down; stays at highest point.

Reset by shaking, but be sure to shake it so that the mercury is forced down into the bulb, not away from it, as once mercury separates it ususally is not possible to get it to rejoin!

Very reliable, and is generally the official means to measure maximum temperatures in the U.S., Vietnam, etc.

2.2.3. Minimum thermo meter works on different principle; has index, so fluid can go up and around it, but as temperature decreases the index is pulled down. Given this, measure the UPPER end of the index, not the lower end.

Again this is the standard method worldwide.

2.2.4. Timing of readings: Minimum and maximum give you the minimum and maximum values since the time the thermometer was last reset.

Have to be careful in terms of when you reset them; if you reset the maximum when it is still warm and the nex day is colder, it will only give you the temperature you reset it at;

If you reset the minimum thermometer while it is cold and the next day is warmer, again it will stay at the temperature when it was reset; hence you want to reset the maximum thermometer at a time when you know it will get warmer (e.g., shortly after sunrise), and the minimum thermometer at a time when you know it will get colder (e.g., in the afternoon);

2.3. Continuous temperature recordings: To get continuous temperature measurements, need either a mechanical thermometer with a chart or an electronic thermometer.

2.3.1. Mechanical recording thermometers (“thermograph”) also work on principle of expansion, as they typically use two pieces of metal with different expansion coefficients; change in temperature causes differential bending, and this change in angle is converted by a system of rods and levers into a rise and fall on a chart. A mechanical or battery-powered clock is used to drive the chart, and the rate at whch the chart moves determines the resolution of the temperature measurements in time; can range from 6 hours to 30 days depending on which combination of gears are used for the clock drive and at the base of the chart; Cost around $500;

2.3.2. Electronic thermometers are now very reliable and cheap, so they are the dominant means for recording temperatures. These only need a minimal power supply, a clock, a temperature sensor (voltage), and a data logger. Can purchase single-channel temperature loggers for $20 or less; now use these on refrigerated trucks to check temperatures while shipping fruit or vegetables across the country or across oceans; Need computer to download the data.

Relative humidity

3.1. Wet/dry bulb methods compare the difference in temperatures between a themometer in dry air and a thermometer with a wet bulb (usually a regular thermometer with a small cotton sock on the bulb. The cotton sock is wetted, preferably using deionized water to avoid mineral build up as the water evaporates.

Evaporation from the wet bulb decreases the temperature relative to the dry bulb, and the difference in temperature between the wet and dry bulb thermometers are inversely proportional to the relative humidity (i.e., lower relative humidity leads to a greater temperature difference, while higher relative humidity leads to a smaller temperature difference).